Thermal image generating device

ABSTRACT

A thermal image producing device comprising an array of separately addressable thin-film resistors on a substrate with a heat sink at its near face and, so as to render the heat flux flowing through the substrate into the heat sink spatially and temporarily uniform, e.g. to avoid smearing, an array of compensating resistors, each beneath a respective one of the image producing resistors and separated therefrom by an insulating layer. Each compensating resistor is controlled so that, it and its associated image producing resistor together, produce a uniform total heat flux.

BACKGROUND AND SUMMARY OF THE INVENTION

Thermal image producing devices typically use a control system whichselectively heats different areas of a screen. U.K. Patent SpecificationNo. 2117957 discloses one such thermal image generating device for usein developing and testing infra-red radiation sensitive electro-opticalequipment. This device has a substrate supporting an array of resistorelements which can be selectively heated by the passage of electricalcurrent therethrough. By selectively heating the resistors, the deviceproduces a controllable thermal image made up of pixels which correspondto respective ones of the resistor elements.

At the rear of the substrate, a heat sink is provided to receive anddissipate the heat produced by the resistors. A problem with such adevice is that, since the temperatures of the elements are not all thesame (and this is necessary so as to produce an image), the heat fluxflowing into the heat sink is not spatially uniform. Moreover, since theimage varies with time, the heat flux flowing into the heat sink at onepoint on its surface will also vary with time.

An object of the invention is to provide a thermal image generatingdevice comprising a resistor array from which the produced heat flux isat least more spatially and temporally uniform.

According to one aspect of the present invention, there is provided athermal image producing device that has an array of first and secondresistor elements and electrical current supply means connected to theelements for enabling them to be selectively heated by the passage ofelectrical current therethrough. The elements are arranged such thatheat produced by the first elements is receivable from the device as acontrollably variable thermal image while the heat produced by thesecond elements renders the distribution of the heat flux produced byall the elements substantially uniform over the area of the array.

According to a second aspect of the invention there is provided athermal image producing device that has a first planar array of resistorelements, electrical current supply means connected to the elements ofthe first array for enabling them to be selectively heated by thepassage of current therethrough and thereby to produce a controllablyvariable thermal image at one side of the array, a second planar arrayof resistor elements positioned at the other side of the first arraywith each element of the second array adjacent a respectivecorresponding one of the elements of the first array but insulated fromsaid respective corresponding one element of the first array, and fromsaid one side of the first array, by dielectric material; and coolingmeans for receiving heat from the said other side of the first array andfrom said second array, the electrical current supply means beingfurther connected to the elements of the second array for enabling themto be selectively heated by the passage of current therethrough torender the distribution of the heat flux produced by all the elementssubstantially uniform over the area of the arrays.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention, reference will be made, byway of example, to the accompanying drawings, in which:

FIG. 1 is a perspective view of part of a thermal image producingdevice,

FIG. 2 is a plan view including one pixel resistor of the device,

FIG. 3 is a partly diagrammable sectional view on line 3--3 in FIG. 2,

FIG. 4 is a circuit diagram of part of the FIG. 1 device and its driveelectronics, and

FIG. 5 is a circuit diagram for illustrating a modification of the FIG.1 device.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The device shown in drawing FIG. 4 of UK Patent Specification No.2117957 comprises a two-dimensional planar array of thin-film resistorsmounted on respective dielectric portions which are in turn mounted on asubstrate with a bus-bar structure arranged between the dielectricportions and the substrate. The bus-bar structure comprises a matrix ofcolumn and row bus-bars, each resistor being connected between one rowand, via a series diode, one column bus-bar. By applying appropriatecurrents to the column bus-bars while strobing the row bus-bars, theresistor elements may be selectively heated to produce a controllablyvariable thermal image. In a modification shown in FIG. 7 of the priorspecification, the bus-bars are used to address and control thepotential stored on a matrix of capacitors which, in turn, control thecurrents flowing through respective ones of the resistor elements. Thecapacitors thus act as latching elements which maintain the temperaturesof the resistor elements between successive updates thereof.

In the device to be described herein, for each resistor element whichforms part of the image producing array, i.e. for each `emitter`resistor, there is provided a second compensating resistor similar (ifnot identical) to the emitter resistor but positioned on the substratedirectly below the emitter resistor and separated from it by adielectric layer. The resistors are independently addressed so that thetotal power dissipated is constant: this is controlled by the externaldrive electronics. To a first order approximation the compensationtechnique gives the following advantages:

(i) The total heat flux leaving the rear of the substrate is uniform andconstant; i.e. there is no variation over the area of the device or withtime.

(ii) The uniform heat flux means that no lateral temperature gradientswill exist even if a large temperature gradient vertically into the heatsink exists. (In the uncompensated case the heat flux was temporally andspatially varying).

(iii) The spatial uniformity allows a large vertical temperaturegradient to exist without thermally smearing the displayed image thusheat sinking across a metal heat sink to a large finned area in forinstance, liquid Nitrogen could be used. Instead of the latter form ofcooler, one which makes use of nucleate boiling could be used, but thisis not preferred because nuclear boiling may impose a peak heat fluxthus limiting the maximum display temperature for a given time response.

The image generating device of FIGS. 1 to 4 comprises a deeply finnedMolybdenum heatsink 1 supporting a silicon substrate 2. The substratesupports a structure including a series of parallel row bus-bars 3, aseries of parallel column bus-bars 4 and 5 which are orthogonal to therow bus-bars so that the bus-bars form a matrix, and two arrays ofthin-film resistors 6 and 7. The resistors 6 are exposed at the outersurface of the structure and are used to produce the thermal image. Theyare connected to respective ones of the column bus-bars 4 and, via arespective diode 8, to a respective one of the row bus-bars 3. Thus, viathe bus-bars 3 and 4, the resistors 6 can be selectively addressed andheated by the passage of electrical current therethrough to produce therequired image. Meanwhile, the resistors 7 lie beneath respective onesof the resistors 6, separated therefrom by an insulation layer 9 ofPolyimide, and are each connected between that respective column bus-bar5 which is adjacent to the bus-bar 4 connected to the overlying resistor6 and, via a diode 10, to the respective bus-bar 3 coupled to thatoverlying resistor 6. Thus, via the bus-bars 3 and 5, the resistors 7can also be selectively addressed and supplied with electrical current.

Referring to FIGS. 2 and 3, the substrate 2 is provided with a surfacelayer 11 of oxide dielectric and the diodes 8 and 10 are integrated intothe substrate. The row bus-bars 3 are deposited on layer 11. For eachassociated pair of the resistors 6 and 7, i.e. each resistor 6 and theunderlying resistor 7, an aperture 12 is provided in the row bus-barsand two connection pads 13 and 14 are provided on layer 11 within theaperture.

The row bus-bars and the pads 13 and 14 are connected to the underlyingstructures of the diodes 8 and 10 via holes 15 in the oxide layer 11.The diodes 8 and 10 are illustrated diagrammatically. The choice ofphysical structure and the means for forming them, and any consequentmodification of the row bus-bar and pad geometry, are all design detailswithin the capabilities of those skilled in the art. By way of example,each diode might be fabricated, in an epitaxial n- layer grown on a p-substrate, within a reverse-biased `bucket diode` formed by anencircling p region and the underlying p- substrate, the function of thebucket diode being to electrically isolate the diodes 8 and the diodes10 one from another. Each diode 8 and 10 could have a central anodecontact and several cathode contacts around it. Then, of course, thedetailed geometry of the pads and row bus-bars will need appropriateadaptation from what is shown in FIG. 3.

First and second relatively thin layers 16 and 17 of polyimide aredeposited over the bus-bars 3, pads 13 and 14 and oxide layer 11 and themuch thicker polyimide layer 9 is provided over the layers 16 and 17.The column bus-bars 4 and 5 are formed on the first polyimide layer 16along with, for each pair of resistors 6 and 7, two connection tags 18and 19 which connect by way of `vias` 20 and 21 respectively (throughconnection for example placed through holes as shown) to respective onesof the pads 13 and 14. The resistors 6 and 7, formed by etching ofrespective layers of titanium, are provided on the outer surfaces ofrespective ones of the polyimide layers 9 and 17. They are connected,one side of each to a respective one of the corresponding pair ofadjacent column bus-bars 4 and 5 and the other side of each to arespective one of the corresponding two tags 18 and 19, by way of vias22, 23, 24 and 25. The holes for the vias 20 to 25 could be formed byplasma etching.

As shown in FIG. 4, the row bus-bars 3 are connected by way ofrespective amplifiers 26 to respective ones of a series of row selectionoutput terminals 33 of a control circuit 38 which has two further outputterminals 29 and 30 connected to respective ones of a gating arrangement31 and a buffer store 32. Image signals, say from an image frame storefed by a television camera (not shown) viewing a scene to be reproducedas an image or from an image generating computer (not shown), arereceived by gating arrangement 31 and, on command from control circuit28, are passed onto the buffer store is to allow for the likelihood thatthe frame update rate of the image signal providing apparatus, forinstance the t.v. camera or computer, will be much slower than the bestupdate rate for the thermal image producing device. Thus, the bufferstore content may be updated via the gating arrangement 31 at the framerate of the signal providing apparatus, for instance every 25milliseconds, and then read out from the buffer store 32 to control theimage generator many times within each 25 millisecond period. Thecontent of store 32 is read, row-by-row, into a register 33 under thecontrol of circuit 28,. Each digital pixel signal of the row held in theregister 33 is fed to a respective one of a plurality of devices 34, onefor each of the column bus-bars 4 and 5, which produce respectiveresistor drive currents corresponding to the digital pixel signals. Thedrive currents are fed via amplifiers 35 to the column bus-bars 4 and 5.The resistors 6 and 7 are separately addressed so that they alwaysdissipate the same total power. Meanwhile, the circuit 28 applies apotential, via the appropriate one of its terminal 27 and the associatedamplifier 26, only to the row bus-bar 3 which corresponds to the imagepixel row held in the register 33, so as to turn on the diodes 8 and 10connected to that row bus-bar. The potential on the other row bus-barsat this time is made such as to keep the diodes connected thereto turnedoff. Thus, the resistors 6 and 7 of the corresponding image generatingarray row pass the respective drive currents, ie this row is updated.After any row has been updated, the next image signal row is read intothe register 33 and the corresponding next row bus-bar has the diodeturn on potential applied thereto. When all the rows have been updated,the sequence is repeated. As noted, the sequence of updating thegenerated thermal image may occur many times for each update of theimage frame held in the buffer store 32.

An alternative embodiment utilizing latching circuits is shown in FIG.5. Each pixel circuit comprising resistors 6 and 7 connects to acorresponding row bus-bar 50 and two column bus-bars 51 and 52 asbefore, but now also receives drive power from drive rails 53 and 54.The emitter resistor 6 is connected between rail 53 and the collector oftransistor 56 of which the base is connected to be controlled by thepotential stored on capacitor 57. That potential is in turn updated,each time the row bus-bar is strobed, by the signal applied viatransistor 58 from column bus-bar 51. A similar circuit controls thecurrent flowing through the compensating resistor 7.

Even if the resistors 6 and 7 in the described embodiments alwaysdissipate the same total power there can still be transient imbalances.For example suppose that at t=o, the power in resistor 6 is zero (henceresistor 7 dissipates P) and then a step function change reverses thisR_(G) →D_(max) R₇ →0 the time constant for heat to reach the substratefrom R₆ is longer than from R₇, since R₇ is closer to the heat sink 1.The effect of R₇ changing therefore is seen by the substrate before theincrease in flux from R₆ ; thus at the changeover there will be atransient drop in the flux in the substrate.

In order to reduce the thermal transient, the signal to R₇ can bedelayed by a time equivalent to the response of R₆. A simple first orderelectronic filter incorporated at each pixel would suffice to do this.It may also be possible to incorporate this response shaping in thesignal handling external to the device.

A further advantage of the uses of the compensating resistors is that atconstant power eases the design of the drive electronics, with the priorart devices disclosed in UK Patent Application No. 2117957A to drive thedevice it may be necessary to have the electronics dump compensationpower into dummy loads.

We claim:
 1. A thermal image producing device comprising:a first arrayof resistor elements; a second array of resistor elements; a heat sink,adjacent which said first and second arrays are mounted, such that saidsecond array is closer to said heat sink than said first array; meansfor thermally and electrically insulating between said first and secondarrays; and electrical current supply means; connected to said resistorelements; for selectively heating said elements by passing electricalcurrent therethrough, wherein the elements are arranged such that heatproduced by elements in said first array is receivable from the deviceas a controllably variable thermal image, while heat produced byelements in said second array is such to render the distribution of theheat flux produced by all the elements substantially uniform over thearea of the arrays.
 2. A thermal image producing device comprising:afirst planar array of resistor elements; electrical current supply meansconnected to the elements of the first array for selectively heatingsaid elements by the passage of current therethrough to thereby producea controllably variable thermal image at one side of the array; a secondplanar array of resistor elements positioned at the other side of thefirst array with each element of the second array adjacent a respectivecorresponding one of the elements of the first array; a dielectricmaterial between said first and second planar array for thermally andelectrically insulating each element of said second array from saidrespective corresponding one element of said first array and from saidone side of said first array; and cooling means for receiving heat fromthe said other side of the first array and from said second array, theelectrical current supply means being further connected to the elementsof the second array and includes means for selectively heating saidelements of said second array by the passage of current therethrough torender the distribution of the heat flux produced by all the elementssubstantially uniform over the area of the arrays.
 3. A device as inclaim 1 wherein said resistor elements in said first array are eachrespectively paired with said resistor elements in said second array,and wherein said electrical supply means includes means for separatelyaddressing ones of said pairs of said resistors such that said resistorpair always dissipates a same total power.
 4. A device as in claim 2wherein said resistor elements in said first array are each respectivelypaired with said resistor elements in said second array, and whereinsaid electrical supply means includes means for separately addressingones of said pairs of said resistors such that said resistor pair alwaysdissipates a same total power.
 5. An apparatus as in claim 3 whereinsaid electrical current supply means includes means for delaying controlsignals to resistor elements in said second array relative to controlsignals to elements in said first array, thus compensating for relativeproximities of said arrays to said heat sink.
 6. An apparatus as inclaim 4 wherein said electrical current supply means includes means fordelaying control signals to resistor elements in said second arrayrelative to control signals to elements in said first array, thuscompensating for relative proximities of said arrays to said coolingmeans.
 7. A device as in claim 5 further comprising a plurality of firstorder electronic filters connected to elements in said second array.